RAID

What

Redundant Array of Independent Disks
A data storage virtualization technology that combines multiple physical disk drive components into one or more logical units for the purposes of data redundancy, performance improvement, or both.

How

When a file system issues a logical O request to the RAID, the RAID internally must calculate which disk (or disks) to access in order to complete the request, and then issue one or more physical I/Os to do so.

RAID Levels

• RAID 0 (Striping):
  • Split data into blocks and written to multiple disks
  • Better IO.
  • No fault tolerance. A disk fails means data loss.
• RAID 1 (Mirroring):
  • Written identically on two disks
  • Same perf.
  • Maximum fault tolerance.
• RAID 2:
  • Uncommon
  • bit-level striping
  • complex error correction code instead of parity
  • cannot serve multiple requests simultaneously
• RIAD 3:
  • Similar to RAID 5, but use byte-level striping and stores parity calculations on dedicated disk.
  • Cannot serve multiple requests simultaneously
• RAID 4:
  • Block-level striping
  • dedicated parity disk
  • requires at least 3 disks and is prone to botlenecks when storing parity bits for each data block on a single drive
• RAID 5:
  • Distributes data and parity information across all disks
  • provides fault tolerance
  • improved read performance
  • slower write perf due to parity calculations
• RAID 6:
  • Similar to RAID 5, but is uses two sets of parity information
  • better fault tolerance in case of 2 simutaneous disk failures
• RAID 10 (1+0):
  • Mirroring data across 2 sets of striped disks
  • Both fault tolerance and improved performance
• RAID 50 (5+0):
  • striping data across multiple RAID 5 arrays
  • improved perf and fault tolerance

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